Continuing development of solid state lighting (SSL) devices such as light emitting diodes (LEDs) or LED arrays, as well as organic LEDs (OLEDs), for low and high power general illumination applications has introduced a need to limit power within predetermined limits to these devices to mitigate or reduce over-temperature or abnormal operation of these SSL devices. Such a power limit capability is desirable under internal or external fault conditions, transient load conditions, as well as ambient temperature conditions.
In terms of fault conditions, OLEDs may have failure mechanisms that include an increase in impedance. In this failure mode, the increased impedance results in increased power dissipation for a fixed LED drive current which may, however, increase the risk of the SSL device being a fire hazard.
In terms of ambient temperature conditions, low and high power LEDs vary in voltage drop with operating junction temperatures whereby as the junction temperature increases, the voltage drop across the LED is reduced. Conversely, if the junction temperature is reduced, the voltage drop across the LED is increased.
For instance, in a cold ambient operating temperature such as −40° C., the LED voltage drop at power up is highest during initial cold start conditions. This results in a higher initial power requirement during turn on and operation until the voltage drop across the LED drops as its junction temperature increases. Cold start operating requirements and corresponding power requirements can exceed predetermined power limits such as defined by UL1310 Class 2 power levels of 100 watts or any other predetermined value.
Approaches exist to limit power and current to an LED load as a result of an external failure of the LED load or an internal failure of a component within an LED power source. One common approach is to disable or turn off the LED power source if an over power or over current condition is detected. Another approach is to implement a “hiccup” mode whereby the output of the LED power source is continually cycled on and off in an over power or over current condition. However, it is not always ideal to immediately disconnect power to a LED load under transient conditions caused by a potential fault, an AC mains transient, a load transient or a temporary overload condition.
Therefore, there is provided a novel apparatus and method for monitoring and limiting power to SSL devices.